Journal of Microscopy, 2014
doi: 10.1111/jmi.12148
Received 18 January 2014; accepted 18 May 2014
Synthesis and characterization of Ag–Co–Ni nanowires R. K. RAI, M. PRATAP SINGH & C. SRIVASTAVA Department of Materials Engineering, Indian Institute of Science, Bangalore, India
Key words. Ag–Co–Ni, electrodeposition, electron microscopy, nanowires. Summary This work provides an electrodeposition-based methodology for synthesizing multicomponent nanowires containing Ag, Co and Ni atoms. Nanowire morphology was obtained by using an anodic alumina membrane with cylindrical pores of 200-nm diameter. Structural, compositional and magnetic characterization revealed that the as-synthesized nanowires adopted a core–shell microstructure. The core (axial region) contained pure Ag phase volumes with a plate-like morphology oriented perpendicular to the nanowire axis. The shell (peripheral region) contained pure Ag nanoparticles along with superparamagnetic Co and Ni rich clusters.
Introduction Functionalities in multicomponent nano-solids essentially originate from a synergistic effect of size, microstructure and properties of individual components (Bailey & Nie, 2003; Zeng & Sun, 2008; Zeng et al., 2011; Kempa et al., 2013). Microstructural design in multicomponent nano-solids therefore can be used as a tool to derive specific functionalities. One way of tailoring the microstructure while keeping the system size and composition constant is adopting a templatebased synthetic methodology in which both growth kinetics and sequence of attachment of the component element atoms to the solid growing inside the template pore can be synergetically controlled. One synthetic methodology that provides both these flexibilities is electrodeposition (Chraibi et al., 2001; Dolati et al., 2003; Liang et al., 2012; Srivastava & Mundotiya, 2012; Bicelli et al., 2013; Li et al., 2013; Lu et al., 2013; Srivastava & Rai, 2013; Wang, 2013). Use of a template attached to the cathode in the electrodeposition process in which the solid grows inside the template pores has produced nano-solids of different sizes and morphologies (Li et al., 2013; Srivastava & Rai, 2013; Wang, 2013). Electrodeposition process variables such as pH, current density, deposition temperature, etc. have typically been varied to control the nucleation and growth process of the deposit in order to generate variety of nonequilibCorrespondence to: Chandan Srivastava, Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India. Tel: +91-80-2293-2834; fax: +91-080-23600-0472; e-mail: [email protected] C 2014 The Authors C 2014 Royal Microscopical Society Journal of Microscopy
rium microstructures (Srivastava & Mundotiya, 2012; Bicelli et al., 2013; Lu et al., 2013) and addition of organic compounds in the plating bath has been used to selectively alter the deposition potential and thus control the deposition rate of a particular element atoms that forms complex with the additive (Chraibi et al., 2001; Dolati et al., 2003; Liang et al., 2012). In this work, template-based electrodeposition process was employed to produce core–shell nanowires containing Ag, Co and Ni atoms. The literature contains extremely limited reports on synthesis and characterization of nano-solids containing Ag, Co and Ni atoms (Yang et al., 2013; Qiu et al., 2014). In one study, Yang et al. (2013) have synthesized core–shell nanoparticles containing Ag atoms in the core and Co–Ni atoms in the shell region. It was shown by Yang et al. (2013) that these core–shell nanoparticles exhibit high catalytic activity towards the hydrolytic dehydrogenation of ammonia borane. In a different study, Qiu et al. (2014) have synthesized triple layered Ag-Co-Ni nanoparticles and investigated their catalytic property. They (Qiu et al., 2014) illustrated that the triple layered nanoparticles exhibit very high activity for the catalytic dehydrogenation of NH3 BH3 . Qiu et al. (2014) explained that the synergetic interaction between Co and Ni in the triple layered nanostructure was responsible for the high catalytic activity. Apart from the above reported studies that illustrate the superior catalytic activity of Ag–Co–Ni nanosolids, one other area where nano-solids containing Ag, Co and Ni atoms can have potential application is magnetic devices. It should be noted that Ag–Co (Karakaya & Thompson, 1986) and Ag–Ni (Singleton & Nash, 1987) systems exhibit negligible miscibility into a solid solution structure due to a very large (>14%) difference in atomic sizes and high positive enthalpy of mixing. Co–Ni system, on the other hand, exhibits miscibility over the entire composition range (Nishizawa & Ishida, 1983). A system that contains Ag, Ni and Co atoms therefore would possibly adopt a microstructure in which Ni-rich and Co-rich ferromagnetic phase volumes are separated by conducting Ag phase. Such configurations where two ferromagnetic phases are separated by a conducting nonmagnetic phase are ideal candidates for magnetoresistance property (Parkin, 1995). In addition to the above identified functionalities, nanowire morphology as obtained in this study also possess a geometrical significance in device fabrication.
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Fig. 1. A representative (A) SEM micrograph and (B) low magnification TEM bright field image of as-synthesized nanowires.
Experiment Three component Ag–Ni–Co nanowires were prepared by the conventional electrodeposition technique. Anodic alumina membrane having cylindrical pores with an average diameter of 200 nm, a nominal thickness of 60 μm and density of 109 pores/cm2 was used as template for obtaining nanowire morphology. A platinum foil was used as anode. A copper foil attached to the alumina disc using an adhesive tape was used as cathode. Electrolyte used was prepared by dissolving 0.169 g of AgNO3 , 2.52 g of Ni(NO3 )2 ·6H2 O, 1.25 g of Co(NO3 )2 ·6H2 O, 0.25 g of H3 BO3 and 1.95 g of thiourea in 100 mL of distilled water. For electrodeposition, a current of 3 mA was applied using a DC power source for 30 min. Current density on the cathode was 10 μA cm−2 . Current density was determined by taking into account the combined area of the base and the pore walls of the gold-coated alumina template. Deposition was carried out at ambient temperature and under argon atmosphere. After the experiment, the alumina template containing nanowires was immersed in 1M NaOH solution for 3–4 h to dissolve the alumina and release the nanowires. as-Synthesized nanowires were washed several times in distilled water for further analysis. Average composition and morphology of the as-synthesized samples were determined by using a Quanta ESEM scanning electron microscope (SEM) fitted with an energy dispersive spectroscopy (EDS) detector. Voltage and current values used for the SEM-based analysis respectively were 25 keV and 85 μA. A 300 keV field emission FEI Tecnai F-30 transmission electron microscope (TEM) was used for obtaining bright field images, selected area electron diffraction (SAD) patterns and compositional information from as-synthesized nanowires. Samples for the TEM-based analysis were prepared by dropdrying a highly dilute dispersion of as-synthesized nanowires on an electron transparent carbon coated Cu grid. Scanning
transmission electron microscopy-EDS technique, which uses a 2-nm-sized electron probe was used for obtaining compositional line profiles from individual nanowires. Room temperature magnetic characterization of the as-deposited nanowires was performed using Lakeshore vibrating sample magnetometer using a field of 2 Tesla. Result and discussion Representative SEM micrograph of the deposit obtained after the dissolution of alumina template is shown in Figure 1(A). Figure 1(A) revealed that the electrodeposition experiment was successful in producing good yield of uniform nanowires. Diameter of the nanowires was measured to be approximately 200 nm, which is expectedly equal to the diameter of the cylindrical pores in the alumina template. Average composition of the nanowires determined from the SEM-EDS analysis was 36 at% Ag, 36 at% Co and 28 at% Ni. A low magnification TEM bright field image of as-synthesized nanowires is shown in Figure 1(B). Figure 1(B) reveals a clear difference in diffraction contrast between axial and peripheral regions of the nanowires, which indicates presence of two separate phases in a core–shell configuration. Scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) image of a representative nanowire is provided in Figure 2(A). A clear difference in image contrast between core and shell regions of the nanowire can be observed in Figure 2(A). Image contrast in the STEM-HAADF image is proportional to the average atomic number in an analysis column (sample thickness) along the electron beam direction (Aert et al., 2011). A difference in image contrast in a STEM-HAADF image therefore represents regions with differences in average composition. As Ag is heavier than Ni, therefore a relatively brighter image contrast for the axial (core) region and a duller image contrast for the peripheral (shell) region strongly indicate that C 2014 The Authors C 2014 Royal Microscopical Society Journal of Microscopy
SYNTHESIS AND CHARACTERIZATION OF Ag–Co–Ni NANOWIRES
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Fig. 2. (A) STEM-HAADF image of a representative nanowire, (B) compositional line profile analysis result. Insert shows the region (line AB) from which the data were obtained.
Fig. 3. (A) TEM bright field image of a nanowire, (B) SAD pattern obtained from the nanowire in B, (C) TEM dark field image obtained from the spot marked ‘A’ in the SAD pattern and (D) TEM dark field image obtained from a portion of the (111) ring in the SAD pattern.
the axial region of the core–shell nanowires in relatively richer in Ag than the peripheral region. This qualitative conclusion was confirmed by compositional line profile analysis of assynthesized nanowires. A representative compositional line profile analysis result is provided in Figure 2(B). Figure 2(B) is a plot of the counts in the Agk , NiK , and CoK characteristic X-ray peaks in the EDS spectrum obtained from different points along the line AB (see insert image in Fig. 2B), which runs along the nanowire diameter. Observations that can be made from Figure 2(B) are: (a) an abrupt increase in the Ag C 2014 The Authors C 2014 Royal Microscopical Society Journal of Microscopy
X-ray counts in the EDS spectrums obtained from the points in the core (axial) region of the nanowire. This observation supported the conclusion derived from the STEM-HAADF image analysis that the core (axial) region of the nanowire is Ag-rich, (b) a fairly constant X-ray counts from Co and Ni atoms along the nanowire diameter. This observation indicated that the peripheral region (shell) of the nanowire is composed to Co and Ni atoms and also the central region of the nanowire contained nearly pure Ag phase, and (c) a nonzero Ag X-ray counts in EDS spectrums obtained from the peripheral (shell) region of
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Fig. 4. Magnetic hysteresis loop obtained from as-synthesized nanowires.
the nanowire revealed the presence of Ag along with Co and Ni in the peripheral (shell) regions of the nanowire. This is also indicated by the STEM-HAADF image in Figure 2(A), which reveals bright contrast Ag nanoparticles (pointed by arrow in Fig. 2A) in the peripheral region of the nanowire. Structure(s) of phases present in the nanowire microstructure were determined from the SAD pattern analysis. SAD pattern obtained from a representative nanowire (shown in Fig. 3A) is provided in Figure 3(B). The SAD pattern shows broad rings and bright spots lying on the rings. Measurement of the interplanar spacing of the crystallographic planes corresponding to the rings and spots revealed that the SAD pattern contained diffraction signatures corresponding only to pure Ag phase. This indicated an existence of pure Ag phase both with random and specific orientation in the nanowire microstructure. Random orientation was represented by the broad rings and preferred orientation was represented by the bright spots in the SAD pattern. TEM dark field image obtained from the spot marked ‘A’ in the SAD pattern is shown in Figure 3(C). Figure 3(C) revealed that the axial region (core) of the nanowire is made up of pure Ag phase volumes with a plate-like morphology oriented perpendicular to the nanowire axis. The plate-like morphology caused streaking of the diffraction spots in the SAD pattern in Figure 3(B). Dark field image in Figure 3(C) supported the compositional analysis result that the core of the nanowire contains pure Ag phase. TEM dark field image obtained from a portion of the (111) diffraction ring in the SAD pattern is shown in Figure 3(D). Dark field image presented in Fig. 3(D) revealed the presence of randomly oriented pure Ag nanoparticles in the peripheral (shell) region of the nanowire. Figure 3(D) supported the compositional analysis result that revealed the presence of Ag nanoparticles in the peripheral (shell) region of the nanowire. Note that the axial (core) region of the nanowire is dark in Figure 3(D), which
Fig. 5. A representative schematic showing the distribution of phases in the microstructure of as-synthesized Ag–Co–Ni nanowire.
indicates absence of randomly oriented Ag nanoparticles in the axial (core) region of the nanowire. Presence of diffraction signatures corresponding only to pure Ag phase in the SAD pattern indicated that the volumes containing Co and Ni atoms in the peripheral (shell) region of the nanowire were either amorphous or in the form of atomic clusters. Presence of Co-rich and Ni-rich phases in the nanowire microstructure was indicated by magnetic measurements. Magnetic hysteresis loop obtained from the as-synthesized nanowires is shown in Figure 4. Negligible magnetic coercivity and absence of magnetic saturation that are typical indicators of superparamagnetism (Knobel et al., 2008) confirmed that the peripheral (shell) region of the nanowire contains atomic clusters or very fine particles composed of ferromagnetic Co and Ni atoms. For easy visualization, a schematic showing the distribution of phases in the microstructure of as-synthesized Ag–Co–Ni nanowire is provided in Figure 5. Conclusion In this work, 200-nm-diameter nanowires composed of Ag, Co and Ni atoms were electrodeposited using an anodic alumina template. Structural, compositional and magnetic characterization of the as-synthesized nanowires revealed a core–shell C 2014 The Authors C 2014 Royal Microscopical Society Journal of Microscopy
SYNTHESIS AND CHARACTERIZATION OF Ag–Co–Ni NANOWIRES
microstructure for the nanowires. Core of the nanowires contained pure Ag-phase volumes in a plate-like morphology with a preferred orientation. Shell of the nanowire contained randomly oriented pure Ag nanoparticles along with Ni-rich and Co-rich superparamagnetic clusters. Acknowledgements Authors thank the electron microscopy facilities available at Advanced Facility for Microscopy and Microanalysis (AFMM), Indian Institute of Science, Bangalore, India. The funding from the Science and Engineering Research Board (SERB) Government of India is deeply acknowledged. References Aert, S.V., Batenburg, K.J., Rossell, M.D., Erni R. & Tendeloo, G.V. (2011) Three-dimensional atomic imaging of crystalline nanoparticles. Nature 470, 374–377. Bailey, R.E. & Nie, S.M. (2003) Alloyed semiconductor quantum dots: tuning the optical properties without changing the particle size. J. Am. Chem. Soc. 125, 7100–7106. Bicelli, L.P., Bozzini, B., Mele, C. & Urzo, L.D. (2013) A review of nanostructural aspects of metal electrodeposition. Int. J. Electrochem. Sci. 3, 356–408. Chraibi, F., Fahoume, M., Ennaoui, A. & Delplancke, J.L. (2001) Influence of citrate ions as complexing agent for electrodeposition of CuInSe2 thin films. Phys. Status Solidi 186(3), 373–381. Dolati, A.G., Ghorbani, M. & Afshar, A. (2003) The electrodeposition of quaternary Fe–Cr–Ni–Mo alloys from the chloride-complexing agents electrolyte. Part I. Processing. Surf. Coat. Tech. 166, 105–110. Karakaya, I. & Thompson, W.T. (1986) The Ag-Co (silver-cobalt) system. Bull. Alloy Phase Diagr. 7(3), 259–260. Kempa, T.J., Kim, S.K., Day, R.W., Park, H.G., Nocera, D.G. & Lieber, C.M. (2013) Facet-selective growth on nanowires yields multi-component nanostructures and photonic devices. J. Am. Chem. Soc. 135, 18354– 18357.
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Knobel, M., Nunes, W.C., Socolovsky, L.M., Biasi, E.D., Vargas, J.M. & Denardin, J.C. (2008) Superparamagnetism and other magnetic features in granular materials: a review on ideal and real systems. J. Nanosci. Nanotechnol. 8, 2836–2857. Li, J., Hu, L., Li, C. & Gao, X. (2013) Tailoring hexagonally packed metal hollow-nanocones and taper-nanotubes by template-induced preferential electrodeposition. ACS Appl. Mater. Interfaces 5(20), 10376–10380. Liang, D., Liu, Z., Hilty, R.D. & Zangari, G. (2012) Electrodeposition of Ag–Ni films from thiourea complexing solutions. Electrochim. Acta 82, 82–89. Lu, W., Ou, C., Huang, P., Yan, P. & Yan, B. (2013) Effect of pH on the structural properties of electrodeposited nanocrystalline FeCo films. Int. J. Electrochem. Sci. 8, 8218–8226. Nishizawa, T. & Ishida, K. (1983) The Co-Ni (cobalt-nickel) system. Bull. Alloy Phase Diagr. 4, 390–391. Parkin, S.S.P. (1995) Giant magnetoresistance in magnetic nanostructures. Annu. Rev. Mater. Sci. 25, 357–388. Qiu, F., Liu, G., Li, L., et al. (2014) Synthesis of triple-layered Ag–Co–Ni core-shell nanoparticles for the catalytic dehydrogenation of ammonia borane. Chem. Eur. J. 20(2), 505–509. Singleton, M. & Nash P. (1987) The Ag-Ni (silver-nickel) system. Bull. Alloy Phase Diagr. 8(2), 119–121. Srivastava, C. & Mundotiya, B.M. (2012) Morphology dependence of AgNi solid solubility. Electrochem. Solid St. 15(2), K10–K15. Srivastava, C. & Rai, R.K. (2013) Transmission electron microscopy study of Ni-rich, Ag–Ni nanowires. Chem. Phys. Lett. 575, 91–96. Wang, J. (2013) Template electrodeposition of catalytic nanomotors. Faraday Discuss 164, 9–18. Yang, L., Su, J., Meng, X., Luo, W. & Cheng, G. (2013) In situ synthesis of graphene supported Ag–Co–Ni core–shell nanoparticles as highly efficient catalysts for hydrogen generation from hydrolysis of ammonia borane and methylamine borane. J. Mater. Chem. 1, 10016–10023. Zeng, H. & Sun, S. (2008) Syntheses, properties, and potential applications of multicomponent magnetic nanoparticles. Adv. Funct. Mater. 18(3), 391–400. Zeng J., Wang, X. & Hou, J.G. (2011) Colloidal hybrid nanocrystals: synthesis, properties, and perspectives, nanocrystal (ed. by Y. Masuda), InTech, Rijeka, Croatia.
doi: 10.1111/jmi.12148
Received 18 January 2014; accepted 18 May 2014
Synthesis and characterization of Ag–Co–Ni nanowires R. K. RAI, M. PRATAP SINGH & C. SRIVASTAVA Department of Materials Engineering, Indian Institute of Science, Bangalore, India
Key words. Ag–Co–Ni, electrodeposition, electron microscopy, nanowires. Summary This work provides an electrodeposition-based methodology for synthesizing multicomponent nanowires containing Ag, Co and Ni atoms. Nanowire morphology was obtained by using an anodic alumina membrane with cylindrical pores of 200-nm diameter. Structural, compositional and magnetic characterization revealed that the as-synthesized nanowires adopted a core–shell microstructure. The core (axial region) contained pure Ag phase volumes with a plate-like morphology oriented perpendicular to the nanowire axis. The shell (peripheral region) contained pure Ag nanoparticles along with superparamagnetic Co and Ni rich clusters.
Introduction Functionalities in multicomponent nano-solids essentially originate from a synergistic effect of size, microstructure and properties of individual components (Bailey & Nie, 2003; Zeng & Sun, 2008; Zeng et al., 2011; Kempa et al., 2013). Microstructural design in multicomponent nano-solids therefore can be used as a tool to derive specific functionalities. One way of tailoring the microstructure while keeping the system size and composition constant is adopting a templatebased synthetic methodology in which both growth kinetics and sequence of attachment of the component element atoms to the solid growing inside the template pore can be synergetically controlled. One synthetic methodology that provides both these flexibilities is electrodeposition (Chraibi et al., 2001; Dolati et al., 2003; Liang et al., 2012; Srivastava & Mundotiya, 2012; Bicelli et al., 2013; Li et al., 2013; Lu et al., 2013; Srivastava & Rai, 2013; Wang, 2013). Use of a template attached to the cathode in the electrodeposition process in which the solid grows inside the template pores has produced nano-solids of different sizes and morphologies (Li et al., 2013; Srivastava & Rai, 2013; Wang, 2013). Electrodeposition process variables such as pH, current density, deposition temperature, etc. have typically been varied to control the nucleation and growth process of the deposit in order to generate variety of nonequilibCorrespondence to: Chandan Srivastava, Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India. Tel: +91-80-2293-2834; fax: +91-080-23600-0472; e-mail: [email protected] C 2014 The Authors C 2014 Royal Microscopical Society Journal of Microscopy
rium microstructures (Srivastava & Mundotiya, 2012; Bicelli et al., 2013; Lu et al., 2013) and addition of organic compounds in the plating bath has been used to selectively alter the deposition potential and thus control the deposition rate of a particular element atoms that forms complex with the additive (Chraibi et al., 2001; Dolati et al., 2003; Liang et al., 2012). In this work, template-based electrodeposition process was employed to produce core–shell nanowires containing Ag, Co and Ni atoms. The literature contains extremely limited reports on synthesis and characterization of nano-solids containing Ag, Co and Ni atoms (Yang et al., 2013; Qiu et al., 2014). In one study, Yang et al. (2013) have synthesized core–shell nanoparticles containing Ag atoms in the core and Co–Ni atoms in the shell region. It was shown by Yang et al. (2013) that these core–shell nanoparticles exhibit high catalytic activity towards the hydrolytic dehydrogenation of ammonia borane. In a different study, Qiu et al. (2014) have synthesized triple layered Ag-Co-Ni nanoparticles and investigated their catalytic property. They (Qiu et al., 2014) illustrated that the triple layered nanoparticles exhibit very high activity for the catalytic dehydrogenation of NH3 BH3 . Qiu et al. (2014) explained that the synergetic interaction between Co and Ni in the triple layered nanostructure was responsible for the high catalytic activity. Apart from the above reported studies that illustrate the superior catalytic activity of Ag–Co–Ni nanosolids, one other area where nano-solids containing Ag, Co and Ni atoms can have potential application is magnetic devices. It should be noted that Ag–Co (Karakaya & Thompson, 1986) and Ag–Ni (Singleton & Nash, 1987) systems exhibit negligible miscibility into a solid solution structure due to a very large (>14%) difference in atomic sizes and high positive enthalpy of mixing. Co–Ni system, on the other hand, exhibits miscibility over the entire composition range (Nishizawa & Ishida, 1983). A system that contains Ag, Ni and Co atoms therefore would possibly adopt a microstructure in which Ni-rich and Co-rich ferromagnetic phase volumes are separated by conducting Ag phase. Such configurations where two ferromagnetic phases are separated by a conducting nonmagnetic phase are ideal candidates for magnetoresistance property (Parkin, 1995). In addition to the above identified functionalities, nanowire morphology as obtained in this study also possess a geometrical significance in device fabrication.
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Fig. 1. A representative (A) SEM micrograph and (B) low magnification TEM bright field image of as-synthesized nanowires.
Experiment Three component Ag–Ni–Co nanowires were prepared by the conventional electrodeposition technique. Anodic alumina membrane having cylindrical pores with an average diameter of 200 nm, a nominal thickness of 60 μm and density of 109 pores/cm2 was used as template for obtaining nanowire morphology. A platinum foil was used as anode. A copper foil attached to the alumina disc using an adhesive tape was used as cathode. Electrolyte used was prepared by dissolving 0.169 g of AgNO3 , 2.52 g of Ni(NO3 )2 ·6H2 O, 1.25 g of Co(NO3 )2 ·6H2 O, 0.25 g of H3 BO3 and 1.95 g of thiourea in 100 mL of distilled water. For electrodeposition, a current of 3 mA was applied using a DC power source for 30 min. Current density on the cathode was 10 μA cm−2 . Current density was determined by taking into account the combined area of the base and the pore walls of the gold-coated alumina template. Deposition was carried out at ambient temperature and under argon atmosphere. After the experiment, the alumina template containing nanowires was immersed in 1M NaOH solution for 3–4 h to dissolve the alumina and release the nanowires. as-Synthesized nanowires were washed several times in distilled water for further analysis. Average composition and morphology of the as-synthesized samples were determined by using a Quanta ESEM scanning electron microscope (SEM) fitted with an energy dispersive spectroscopy (EDS) detector. Voltage and current values used for the SEM-based analysis respectively were 25 keV and 85 μA. A 300 keV field emission FEI Tecnai F-30 transmission electron microscope (TEM) was used for obtaining bright field images, selected area electron diffraction (SAD) patterns and compositional information from as-synthesized nanowires. Samples for the TEM-based analysis were prepared by dropdrying a highly dilute dispersion of as-synthesized nanowires on an electron transparent carbon coated Cu grid. Scanning
transmission electron microscopy-EDS technique, which uses a 2-nm-sized electron probe was used for obtaining compositional line profiles from individual nanowires. Room temperature magnetic characterization of the as-deposited nanowires was performed using Lakeshore vibrating sample magnetometer using a field of 2 Tesla. Result and discussion Representative SEM micrograph of the deposit obtained after the dissolution of alumina template is shown in Figure 1(A). Figure 1(A) revealed that the electrodeposition experiment was successful in producing good yield of uniform nanowires. Diameter of the nanowires was measured to be approximately 200 nm, which is expectedly equal to the diameter of the cylindrical pores in the alumina template. Average composition of the nanowires determined from the SEM-EDS analysis was 36 at% Ag, 36 at% Co and 28 at% Ni. A low magnification TEM bright field image of as-synthesized nanowires is shown in Figure 1(B). Figure 1(B) reveals a clear difference in diffraction contrast between axial and peripheral regions of the nanowires, which indicates presence of two separate phases in a core–shell configuration. Scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) image of a representative nanowire is provided in Figure 2(A). A clear difference in image contrast between core and shell regions of the nanowire can be observed in Figure 2(A). Image contrast in the STEM-HAADF image is proportional to the average atomic number in an analysis column (sample thickness) along the electron beam direction (Aert et al., 2011). A difference in image contrast in a STEM-HAADF image therefore represents regions with differences in average composition. As Ag is heavier than Ni, therefore a relatively brighter image contrast for the axial (core) region and a duller image contrast for the peripheral (shell) region strongly indicate that C 2014 The Authors C 2014 Royal Microscopical Society Journal of Microscopy
SYNTHESIS AND CHARACTERIZATION OF Ag–Co–Ni NANOWIRES
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Fig. 2. (A) STEM-HAADF image of a representative nanowire, (B) compositional line profile analysis result. Insert shows the region (line AB) from which the data were obtained.
Fig. 3. (A) TEM bright field image of a nanowire, (B) SAD pattern obtained from the nanowire in B, (C) TEM dark field image obtained from the spot marked ‘A’ in the SAD pattern and (D) TEM dark field image obtained from a portion of the (111) ring in the SAD pattern.
the axial region of the core–shell nanowires in relatively richer in Ag than the peripheral region. This qualitative conclusion was confirmed by compositional line profile analysis of assynthesized nanowires. A representative compositional line profile analysis result is provided in Figure 2(B). Figure 2(B) is a plot of the counts in the Agk , NiK , and CoK characteristic X-ray peaks in the EDS spectrum obtained from different points along the line AB (see insert image in Fig. 2B), which runs along the nanowire diameter. Observations that can be made from Figure 2(B) are: (a) an abrupt increase in the Ag C 2014 The Authors C 2014 Royal Microscopical Society Journal of Microscopy
X-ray counts in the EDS spectrums obtained from the points in the core (axial) region of the nanowire. This observation supported the conclusion derived from the STEM-HAADF image analysis that the core (axial) region of the nanowire is Ag-rich, (b) a fairly constant X-ray counts from Co and Ni atoms along the nanowire diameter. This observation indicated that the peripheral region (shell) of the nanowire is composed to Co and Ni atoms and also the central region of the nanowire contained nearly pure Ag phase, and (c) a nonzero Ag X-ray counts in EDS spectrums obtained from the peripheral (shell) region of
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Fig. 4. Magnetic hysteresis loop obtained from as-synthesized nanowires.
the nanowire revealed the presence of Ag along with Co and Ni in the peripheral (shell) regions of the nanowire. This is also indicated by the STEM-HAADF image in Figure 2(A), which reveals bright contrast Ag nanoparticles (pointed by arrow in Fig. 2A) in the peripheral region of the nanowire. Structure(s) of phases present in the nanowire microstructure were determined from the SAD pattern analysis. SAD pattern obtained from a representative nanowire (shown in Fig. 3A) is provided in Figure 3(B). The SAD pattern shows broad rings and bright spots lying on the rings. Measurement of the interplanar spacing of the crystallographic planes corresponding to the rings and spots revealed that the SAD pattern contained diffraction signatures corresponding only to pure Ag phase. This indicated an existence of pure Ag phase both with random and specific orientation in the nanowire microstructure. Random orientation was represented by the broad rings and preferred orientation was represented by the bright spots in the SAD pattern. TEM dark field image obtained from the spot marked ‘A’ in the SAD pattern is shown in Figure 3(C). Figure 3(C) revealed that the axial region (core) of the nanowire is made up of pure Ag phase volumes with a plate-like morphology oriented perpendicular to the nanowire axis. The plate-like morphology caused streaking of the diffraction spots in the SAD pattern in Figure 3(B). Dark field image in Figure 3(C) supported the compositional analysis result that the core of the nanowire contains pure Ag phase. TEM dark field image obtained from a portion of the (111) diffraction ring in the SAD pattern is shown in Figure 3(D). Dark field image presented in Fig. 3(D) revealed the presence of randomly oriented pure Ag nanoparticles in the peripheral (shell) region of the nanowire. Figure 3(D) supported the compositional analysis result that revealed the presence of Ag nanoparticles in the peripheral (shell) region of the nanowire. Note that the axial (core) region of the nanowire is dark in Figure 3(D), which
Fig. 5. A representative schematic showing the distribution of phases in the microstructure of as-synthesized Ag–Co–Ni nanowire.
indicates absence of randomly oriented Ag nanoparticles in the axial (core) region of the nanowire. Presence of diffraction signatures corresponding only to pure Ag phase in the SAD pattern indicated that the volumes containing Co and Ni atoms in the peripheral (shell) region of the nanowire were either amorphous or in the form of atomic clusters. Presence of Co-rich and Ni-rich phases in the nanowire microstructure was indicated by magnetic measurements. Magnetic hysteresis loop obtained from the as-synthesized nanowires is shown in Figure 4. Negligible magnetic coercivity and absence of magnetic saturation that are typical indicators of superparamagnetism (Knobel et al., 2008) confirmed that the peripheral (shell) region of the nanowire contains atomic clusters or very fine particles composed of ferromagnetic Co and Ni atoms. For easy visualization, a schematic showing the distribution of phases in the microstructure of as-synthesized Ag–Co–Ni nanowire is provided in Figure 5. Conclusion In this work, 200-nm-diameter nanowires composed of Ag, Co and Ni atoms were electrodeposited using an anodic alumina template. Structural, compositional and magnetic characterization of the as-synthesized nanowires revealed a core–shell C 2014 The Authors C 2014 Royal Microscopical Society Journal of Microscopy
SYNTHESIS AND CHARACTERIZATION OF Ag–Co–Ni NANOWIRES
microstructure for the nanowires. Core of the nanowires contained pure Ag-phase volumes in a plate-like morphology with a preferred orientation. Shell of the nanowire contained randomly oriented pure Ag nanoparticles along with Ni-rich and Co-rich superparamagnetic clusters. Acknowledgements Authors thank the electron microscopy facilities available at Advanced Facility for Microscopy and Microanalysis (AFMM), Indian Institute of Science, Bangalore, India. The funding from the Science and Engineering Research Board (SERB) Government of India is deeply acknowledged. References Aert, S.V., Batenburg, K.J., Rossell, M.D., Erni R. & Tendeloo, G.V. (2011) Three-dimensional atomic imaging of crystalline nanoparticles. Nature 470, 374–377. Bailey, R.E. & Nie, S.M. (2003) Alloyed semiconductor quantum dots: tuning the optical properties without changing the particle size. J. Am. Chem. Soc. 125, 7100–7106. Bicelli, L.P., Bozzini, B., Mele, C. & Urzo, L.D. (2013) A review of nanostructural aspects of metal electrodeposition. Int. J. Electrochem. Sci. 3, 356–408. Chraibi, F., Fahoume, M., Ennaoui, A. & Delplancke, J.L. (2001) Influence of citrate ions as complexing agent for electrodeposition of CuInSe2 thin films. Phys. Status Solidi 186(3), 373–381. Dolati, A.G., Ghorbani, M. & Afshar, A. (2003) The electrodeposition of quaternary Fe–Cr–Ni–Mo alloys from the chloride-complexing agents electrolyte. Part I. Processing. Surf. Coat. Tech. 166, 105–110. Karakaya, I. & Thompson, W.T. (1986) The Ag-Co (silver-cobalt) system. Bull. Alloy Phase Diagr. 7(3), 259–260. Kempa, T.J., Kim, S.K., Day, R.W., Park, H.G., Nocera, D.G. & Lieber, C.M. (2013) Facet-selective growth on nanowires yields multi-component nanostructures and photonic devices. J. Am. Chem. Soc. 135, 18354– 18357.
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Knobel, M., Nunes, W.C., Socolovsky, L.M., Biasi, E.D., Vargas, J.M. & Denardin, J.C. (2008) Superparamagnetism and other magnetic features in granular materials: a review on ideal and real systems. J. Nanosci. Nanotechnol. 8, 2836–2857. Li, J., Hu, L., Li, C. & Gao, X. (2013) Tailoring hexagonally packed metal hollow-nanocones and taper-nanotubes by template-induced preferential electrodeposition. ACS Appl. Mater. Interfaces 5(20), 10376–10380. Liang, D., Liu, Z., Hilty, R.D. & Zangari, G. (2012) Electrodeposition of Ag–Ni films from thiourea complexing solutions. Electrochim. Acta 82, 82–89. Lu, W., Ou, C., Huang, P., Yan, P. & Yan, B. (2013) Effect of pH on the structural properties of electrodeposited nanocrystalline FeCo films. Int. J. Electrochem. Sci. 8, 8218–8226. Nishizawa, T. & Ishida, K. (1983) The Co-Ni (cobalt-nickel) system. Bull. Alloy Phase Diagr. 4, 390–391. Parkin, S.S.P. (1995) Giant magnetoresistance in magnetic nanostructures. Annu. Rev. Mater. Sci. 25, 357–388. Qiu, F., Liu, G., Li, L., et al. (2014) Synthesis of triple-layered Ag–Co–Ni core-shell nanoparticles for the catalytic dehydrogenation of ammonia borane. Chem. Eur. J. 20(2), 505–509. Singleton, M. & Nash P. (1987) The Ag-Ni (silver-nickel) system. Bull. Alloy Phase Diagr. 8(2), 119–121. Srivastava, C. & Mundotiya, B.M. (2012) Morphology dependence of AgNi solid solubility. Electrochem. Solid St. 15(2), K10–K15. Srivastava, C. & Rai, R.K. (2013) Transmission electron microscopy study of Ni-rich, Ag–Ni nanowires. Chem. Phys. Lett. 575, 91–96. Wang, J. (2013) Template electrodeposition of catalytic nanomotors. Faraday Discuss 164, 9–18. Yang, L., Su, J., Meng, X., Luo, W. & Cheng, G. (2013) In situ synthesis of graphene supported Ag–Co–Ni core–shell nanoparticles as highly efficient catalysts for hydrogen generation from hydrolysis of ammonia borane and methylamine borane. J. Mater. Chem. 1, 10016–10023. Zeng, H. & Sun, S. (2008) Syntheses, properties, and potential applications of multicomponent magnetic nanoparticles. Adv. Funct. Mater. 18(3), 391–400. Zeng J., Wang, X. & Hou, J.G. (2011) Colloidal hybrid nanocrystals: synthesis, properties, and perspectives, nanocrystal (ed. by Y. Masuda), InTech, Rijeka, Croatia.
